Freshly machined metallic (e.g. aluminum) parts, or components, have a highly reflective surface finish such that they can be considered a mirror with random grating marks. Conducting optical metrology on reflective surfaces of this type is difficult because the secondary reflections show up as bright anomalies that can severely complicate analysis. Although the initial measuring spot is visible, the anomalous light caused by scattering, diffraction, reflections, and multiple reflections off the part surface other than from the desired scan region show up as much brighter when recorded by a detector (or camera).
An existing form of non-contact profile measurement system that is currently commercially available includes the use of a laser fan that illuminates the part to be tested and a two-dimensional area detector that measures the profile of the part. This type of system uses no moving parts, includes the ability to operate with background ambient light, and results in cross-section measurement by simultaneously analyzing the entire area illuminated by the laser fan. The system has disadvantages associated with it that are significant, for example, the number of rows and columns in the area detector fundamentally limits depth resolution and cross-section resolution, respectively, of the system. Complicated tradeoffs in imaging performance occur because the area detector is rectangular in extent while the field of view of the area detector and the laser fan are both roughly trapezoidal in shape. This results in the area detector having a practical readout speed limitation of less than 60 frames per second which limits how fast this system can scan parts. The most significant drawback of this scanning technique is the system's sensitivity to spurious reflections from highly reflective parts because, as explained above, the image of spurious reflections of the laser fan can be brighter than the initial image where the-fan illuminates the portion of the part. Such detection of the spurious reflections resulting from the laser striking highly reflective machined surfaces confuses the image processing of this type of scanning system and, therefore, renders the system completely ineffective at measuring highly reflective machined surfaces.
Another existing non-contact profile measurement system that utilizes a laser having a potential for less sensitivity to spurious reflections is a system comprising a single point of illumination light that is scanned across the part and measured using a “staring” area detector having a fixed field of view. Such a system, however, is not immune to detecting spurious reflections and can be easily confused when the image of the spurious light is brighter than the image of the initial laser spot. This system also has limitations to its depth resolution and cross-section resolution dependent on the characteristics of the area detector utilized. The most significant drawback to this particular approach is that it is extremely slow since it can measure only one point per frame of the area detector. This results in the area detector having a practical readout speed limitation of less than 60 points per second.
Even still another existing non-contact profile measurement system that utilizes a laser is a height gauge system which uses a single point laser illumination and a linear detector. There are several inexpensive and relatively fast single point laser scanners based on this technique that are commercially available for applications such as web inspections. Although this type of system has low sensitivity to off-axis spurious reflections due to the linear detector having a limited field of view, the main drawback to this technique is that the system only measures the height of the test part in one location and has no provision to provide a cross-sectional profile scan of the entire external surface of the part. It is possible to move the part under the single point scan or to move the system completely around the external profile of the part. This would be the optical equivalent of the single point touch probes used in coordinate measuring machines (CMMs). Although this technique can be accurate, it is also very slow. Since an excessive amount of time would be required to measure the external surface profile of the part with sufficient density, this technique is normally utilized to measure only a few representative points along the external surface of the part.
Polygon mirrors are well known in the art of applications such as printing and bar-code scanning. These polygon mirror scanners involve a metal disk with highly polished facets around the perimeter. In such implementations, the metal disk acts as both the structure of the rotor and the substrate of the mirror. This monolithic approach can yield a stable structure with very repeatable scan characteristics. One drawback to this method is that the surfaces of the mirrors are prone to defects left over from the machining process. If a post-machining polishing step is used to minimize the mirror defects, other undesirable defects such as edge turndown and wavy surfaces are likely to result. These common defects can result in unwanted scattering and out-of-plane wandering of the reflected light. As a result, high quality monolithic polygon mirror construction is time consuming and is therefore an expensive process that would be extremely prohibitive for the size of the rotor required in this system.
Another typical method for constructing these types of polygon mirror scanners involve adhering individual mirrors (typically first surface glass mirrors) to a supporting rotor structure. This is an inexpensive method of insuring good quality mirror surfaces that can be applied to rotors with large facets. The potential drawback to this technique is that it can be difficult to adhere the mirrors to the substrate in a fashion that insures common alignment of all the facets so as to minimize out-of-plane variation of the reflected light. Additionally, the mechanical stability and alignment can be adversely affected in the presence of effects such as temperature variations.
The most common motors used to drive precision scanner rotors are either AC brushless or DC brushless motors. Brushless motors are utilized primarily because they have minimized rotational “cogging” which is present to a small degree with all brushed motors and to a very large extent with stepper motors. At high rotational speeds, brushless motors can be controlled to yield extremely constant rotational velocities. However, these precision controllers are relatively expensive to implement.
A barrier to utilizing either brushless or brushed motors in this scanning system is that it is difficult to establish precise rotational control when the rotational velocities are as slow as 60 RPM. The torque delivered by such motors during rotation is centered on a few poles determined by the structure of the motors. To help even out the uneven application of torque, angular momentum (L) is typically utilized to smooth out the effects of the uneven forces applied during rotation. The angular momentum is related to the structure and motion of the rotor by L=Iω where I is the moment of inertia of the rotor and ω is the angular velocity. At high angular velocities, the rotor stores a lot of angular momentum and only small amounts of torque per impulse are needed, resulting in minimum perturbation of the velocity. With slow rotation, the rotor does not store much angular momentum, higher torque impulses are needed, and effective control becomes difficult. Additionally, stepper-type motors have been utilized in existing scanning systems. Although a stepper motor is relatively low in cost and is inexpensive to control, this type of motor subjects the rotor that is directly coupled thereto to high frequency impulses from the stepping of the motor. These unwanted high frequency impulses cause significant vibrations which adversely affect the effective control of the rotational velocity of the rotor.
A key part of establishing control of the rotational velocity of a rotor is a method of monitoring either its angular velocity or angular position. Knowledge of angular position is also extremely important in a polar coordinate dimensional scanning application in order to determine exactly what the pointing angle of the measurement beam is when the measurement is taken. One typical method of angular position determination is to use a quadrature signal off a pair of ancillary motor windings to indicate the position of the motor shaft. However, this method cannot practically achieve the 5 μradians resolution needed by this system. Another typical method involves the use of an optical or magnet readout angular encoder affixed to the shaft of either the motor or the rotor. Although angular encoders with sufficient resolution can be obtained, they are extremely cost prohibitive. Further, a shaft encoder only provides indirect information about the actual location of the scan beam. If a mirror facet is not situated perfectly tangential to the rotor radius, e.g. if the mirror facet is twisted, then there would be an angular mismatch between the measurement beam and the reading from the shaft encoder.
An existing method of monitoring the angular velocity of the rotor that is coupled to the actual scanning beam is a timing method based on a start-of-scan (SOS) pulse and an optional end-of-scan (EOS) pulse. Such a method usually utilizes a high-speed clock that is reset when the beam sweeps across the SOS detector. Angular velocity control of the rotor can be achieved by monitoring either the time between successive SOS pulses or, more effectively, the elapsed time between SOS and EOS pulses. As shown in FIG. 11, one common implementation of a SOS detector is a knife-edge aperture with an optical detector 310 situated behind it. The measurement beam sweeps across the aperture and onto the detector, providing a sharp edge in the detected photo-signal to trigger the start of the clock. The potential drawback to this implementation is that the rising edge of the photo-signal has a slope that is related to the width of the laser beam, the scan speed of the beam sweep and the intensity of the measurement beam. The leading edge of the beam clearing the aperture will cause the onset of a rising photo-signal (point A—curve 1) which will continue to rise until the lagging edge of the beam clears the aperture (point B—curve 1). When the photo-signal crosses a fixed trigger level, the clock will start at position θ1. However, for the same beam width and sweep rate, a beam of lesser intensity (shown as dashed curve 2) will trigger the clock at a later position θ2. This variation will cause jitter in the angular reference and degrade the precision of the entire scanning system.
A still further problem with existing non-contact measurement systems that utilize a laser involves eye safety concerns. In order to comply with eye safety requirements, the system needs to incorporate a fail-safe technique for avoiding eye damage caused by the measurement laser when the rotation rate of the rotor falls below a critical threshold. A fail-safe approach must be implemented that prevents the measurement laser beam from direct viewing until the rotor has reached a safe rotational speed and turns it off or blocks it in the event that the rotor stalls or slows below the safe rotational speed. Although a physical shutter mechanism could fill this requirement, it is not easily designed to be fail-safe.
It is therefore an object of the present invention to provide a system which has reduced sensitivity to spurious reflections of highly reflective parts when measuring the profile of an external surface of a component.
It is another object of the present invention to provide a system which has high depth and cross-sectional resolution output capability when measuring the profile of an external surface of a component.
It is a further object of the present invention to provide a system which is capable of high-speed measurement of the entire cross-sectional profile of the external surface of the component when measuring the profile of an external surface of a component.
It is a further object of the present invention to provide a system which has a large depth of field and a large cross-sectional field of view when measuring the profile of an external surface of a component.
It is a further object of the present invention to provide a system that is used for measuring the profile of an external surface of a component and which includes an automatic gain control system which controls the output power of a source of light to thereby avoid saturating the exposure of the sensor.
It is a further object of the present invention to provide a system which is capable of attenuating or eliminating undesired high frequency vibrations associated with the use of, for example, a stepper motor, when measuring the profile of an external surface of a component.
It is a further object of the present invention to provide a system that is used for measuring the profile of an external surface of a component and which includes a rotor structure design having precisely aligned outside reflecting surfaces of mirrors during rotation.
It is a further object of the present invention to provide a system that is used for measuring the profile of an external surface of a component and which monitors and controls the angular velocity or angular position of the rotor in order to control with high precision the rotational velocity of the rotor. Such a system would not suffer from temporal or positional jitter due to variations in the intensity of the measurement beam, variations in laser beam width, and variations in the scan speed of the beam sweep.
It is a still further object of the present invention to provide a system that is used for measuring the profile of an external surface of a component and which includes a fail-safe technique that avoids the potential for eye damage caused by the laser measurement beam.
These and other objects and advantages of the invention will become more fully apparent from the description and claims which follow or may be learned by the practice of the invention.